Photodetectors and their associated sensing circuits have long been known in the art. It has only been relatively recently, however, with the commercial development of laser technology and new uses therefor, that a need has developed for significantly improving the accuracy and response times for photodetector circuitry. This need is most pronounced in the optical recording technology, often referred to as the digital laser recording art. Such technology, which offers the capability for real-time optical recording of image data in highly compressed format and optoelectronic access to the recorded data, provides a practical alternative and often marked advantages over the traditional magnetic recording technology. The optical recording technology is directly compatible with the principles of digital recording and provides a ready framework for computer based document storage and retrieval and record management systems.
While a number of areas utilizing optical data recording principles have been explored, the technology is still in its early stages of development. Typical of such optical data recording systems in use today are those which use a highly focused laser beam for storing (or "writing") the binary digital information on an optically sensitive or responsive storage media, and a lower-powered highly focused laser beam that scans across the recording medium for "reading" the binary information stored thereon. The typical recording medium in use today is an optical storage disk that rotates relative to the "read" and "write" laser heads which are movably controlled so as to appropriately scan across the disk surface as it rotates.
As will be readily understood by those skilled in the art, the "tracks" upon which "data" is written on a disk may already contain bits of prerecorded synchronization and timing information that dictate in part the manner in which subsequent "data" is written onto the tracks. The laser and associated photodetectors associated with the optical "read" sensors in an optical read/write head are, therefore, continuously scanning the disk for such synchronization bits of information as the "data" information is being written onto the disk. The optical read sensors must be sensitive to the detection of the fairly low-intensity optical signals from the "read" laser, and typically include preamplifiers and amplifiers for magnifying the detected optical signals to workable signal strengths for use as control signals for the optical disk drive system that they service. In a typical optical disk drive system, when the optical read sensors detect a large power burst from the "write" laser during a write operation, the read sensor amplifiers typically suffer an input overload of ten or more times that for which they are designed when operating to sense the presence of bits recorded on the media, causing the read amplifiers to saturate and to overcharge filtering networks that receive signals from the read amplifiers. Such overload condition causes significant delays in the response time and effectiveness of the read sensor circuitry since all effects of the overload condition must settle before the read circuitry detection process can resume operation in its intended manner. Such settling process wastes valuable time and can result in the read sensor circuitry being ineffective to detect, for example, subsequent bits of timing information as they pass the read sensor head. This problem has not been adequately addressed by prior art optical disk drive amplifier circuit designs. Besides the requirement for fast overload recovery, it is desirable for the photodetector amplifier circuitry to have excellent DC accuracy and wideband signal handling capability (typically 0-100 MHz). Heretofore, the combination of these features has not been found in the art.
The present invention addresses the above design shortcomings heretofore practiced in the art, by providing a simple yet highly effective, reliable and accurate amplifier design for processing detected signals from photodetectors in a manner which prevents amplifier overloading during detection of a "write" signal, and which provides excellent DC accuracy and wideband signal handling capability. Due to the unique manner in which the amplifier circuitry of this invention processes incoming photodetector signals, transistors within the amplifier are not allowed to saturate when an overload input signal is received, and downstream filter networks which have long time constants are not excessively charged during detection of a write signal. These and other features of the invention will become apparent to those skilled in the art upon a more detailed description herein.